Fluoropolymers display outstanding chemical resistance, solvent resistance, and heat resistance, among other properties, and, therefore, are in use in a great diversity of industrial fields, such as automotive industry, semiconductor industry, chemical industry, etc. as raw materials for sealants to be used under rugged conditions.
Production of fluoropolymers has heretofore been carried out mostly by emulsion polymerization of a fluoroolefin monomer using a water-soluble radical initiator in an aqueous medium or suspension polymerization of a fluoroolefin monomer using an oil-soluble radical initiator. In these polymerization, the reaction field is substantially situated in inside of the polymer particles being produced or in an inert solvent which does not materially affect the polymerization.
The conventional emulsion polymerization using an aqueous medium is generally carried out in the presence of a water-soluble initiator. Owing to this water-soluble initiator, the terminal groups of the polymer produced are rendered ionic and heat-labile, with the consequence that foaming and other troubles are liable to occur in the molding stage. The conventional emulsion polymerization method includes a step in which the aqueous dispersion obtained thereby is subjected to coagulation using an appropriate coagulating agent and to drying to remove water and then recover a solid polymer, therefore the method has another problem that it involves a time-consuming, complicated production sequence which impedes efficient production. Residues of the ionic initiator having run into the product obtained has been another problem in the use of moldings as mechanical parts of semiconductor production equipment.
The suspension polymerization method has the drawback that deposits of the product polymer adhere to the inside wall of the reactor vessel to reduce the polymer yield and add to the cost of polymer production. Furthermore, the suspension polymerization has a further problem: namely the process involves a time-consuming cleaning step for removing the suspension stabilizer used for the polymerization.
Recently much research has been undertaken on the use of a supercritical fluid, chiefly carbon dioxide, as a reaction field. Supercritical fluids feature good heat conductivities, high diffusion rates and low viscosities, thus having characteristics suitable for use as the reaction medium. The supercritical fluid is a fluid within the region transcending both of critical temperature and critical pressure, and generally for economic reasons, a range not transcending the critical point too far tends to be used with preference.
By way of polymerization of fluoroolefin monomers using a supercritical fluid as a reaction field, the specification of JP Kohyo H07-505429, for instance, discloses the radical polymerization of a fluoroacrylate using supercritical carbon dioxide as the reaction field. Moreover, the specification of U.S. Pat. No. 5,312,882 discloses a polymerization, in the presence of a surfactant containing a moiety having an affinity for carbon dioxide, using supercritical carbon dioxide as a continuous phase and a fluoroolefin monomer as a dispersed phase. The specification of U.S. Pat. No. 5,527,865 discloses a radical polymerization of tetrafluoroethylene in a biphasic nonhomogeneous system involving the concurrent use of water and supercritical carbon dioxide in the presence of a fluorine-containing anionic surfactant. Furthermore, the specification of U.S. Pat. No. 5,618,894 describes a technology for the homopolymerization of tetrafluoroethylene, copolymerization of tetrafluoroethylene/perfluoro(propyl vinyl ether), and copolymerization of vinylidene fluoride/hexafluoropropylene, wherein the reaction is conducted under anhydrous conditions using a radical polymerization initiator capable of generating a stable polymer-end-group in supercritical carbon dioxide. In these technologies, the supercritical fluid is invariably restricted to carbon dioxide or a carbon-dioxide-containing mixture. The specification of JP Kohyo H10-502691 discloses the reaction using carbon dioxide, hydrofluorocarbon, perfluorocarbon, or a mixture thereof in the form of a liquid maintained at supraatmospheric pressure or a supercritical fluid. However, the reaction field for this reaction requires an auxiliary dispersing agent as an indispensable component but this requirement is unfavorable from the standpoint of increasing the purity of the product polymer and, moreover, it is not that the reaction substrate fluoroolefin monomer itself is in the form of a supercritical fluid.
As examples of the polymerization of a fluoroolefin monomer which was carried out by transforming the very fluoroolefin monomer into a supercritical fluid and using it as a reaction field are the copolymerization of tetrafluoroethylene/hexafluoropropylene as described in the specification of U.S. Pat. No. 3,062,793, the copolymerization of tetrafluoroethylene/hexafluoropropylene and the copolymerization of vinylidene fluoride/hexafluoropropylene as described in the pamphlet of WO 96/24624. However, the former specification contains no description about VdF and the reaction condition disclosed there is not below about 200 MPa. The reaction conditions mentioned in the latter specification are very rugged high-temperature, high pressure conditions, namely a pressure somewhere between 41 and 690 MPa and a temperature somewhere between 200 and 400° C., with the result that the technology has the drawback of large capital expenditures needed for commercial-scale production.
As the polymerization of a supercritical fluoroolefin monomer at comparatively low temperature and low pressure, the pamphlet of WO 00/47641 discloses the copolymerization of vinylidene fluoride and hexafluoropropylene. However, this pamphlet contains no reference to polymerization at critical-to-supracritical density.